Technology for shifting the phase of the reference signal by a phase difference in accordance with the rotation position of the measured object to output a detection signal and detecting the phase difference between the reference signal and the detection signal to thereby measure the rotation position of the measured object is known.
For example, Patent Document 1 discloses a phase detection device of a 2-phase input/1-phase output resolver. The resolver receives as input two excitation signals fluctuating in their signal levels at sin(ωt) and cos(ωt). The excitation signal is the reference signal or a signal having the same phase as that of the reference signal. The resolver outputs a detection signal obtained by shifting (phase modulating) one of the input excitation signals by a rotation angle θ of a resolver shaft. Namely, the resolver outputs a detection signal fluctuating in its signal level at sin(ωt+θ).
Further, the section of Prior Art of Patent Document 1 discloses technology of detecting a phase difference θ between the excitation signal (reference signal) and the detection signal by providing a counter starting a count up at a rising edge (at a time when t=0) of a signal obtained by digitalizing the excitation signal sin(ωt) and a latch circuit latching the value of the counter at a rising edge of a signal obtained by digitalizing the detection signal sin(ωt+θ).
However, depending on the combination of the counter and the latch circuit as described above, the inconvenience exemplified below arises. When the measured object is rotating, the period of the detection signal fluctuates with respect to the period of the excitation signal and the period of the detection signal sometimes becomes longer than the period of the excitation signal. The faster the rotation velocity of the measured object becomes, the larger the fluctuation amount becomes. Accordingly, when the rotation velocity is fast, the next rising edge (t=2π/ω) of the excitation signal sometimes arrives before the rising edge of the detection signal corresponding to one rising edge (t=0) of the excitation signal is detected. In this case, depending on the combination of the counter and the latch circuit as described above, before the count value indicating the phase signal of the detection signal with respect to above one rising edge (t=0) of the excitation signal is latched, the counter is reset and the measurement of the phase difference of the detection signal with respect to the next rising edge (t=2π/ω) of the excitation signal is started. In order to prevent this, the frequency of the excitation signal is raised.
Therefore, technology providing two counters is known. In this technology, counters are reset for each two cycles of the excitation signal and at timing deviated by one cycle relative to each other. Namely, by detecting one phase difference for each two cycles of the excitation signal by one counter and alternately starting counting of the phase difference for each cycle by two counters, one phase difference is detected for each cycle. Due to this, resetting before the counter value is latched even when the period of the detection signal is long is suppressed and a drop of the frequency of the excitation signal is enabled (note that this technology will be explained later with reference to FIG. 4(a)).
Further, technology not performing the phase difference count by a counter which is reset in synchronization with the excitation signal as described above is also known. For example, technology of detecting the rotation position of a measured object by measuring the period of the detection signal, acquiring information concerning the rotation velocity of the measured object from that period, and adding up the rotation velocities of the measured object is known.    Patent Document 1: Japanese Patent Publication (A) No. 61-137010